1. Field of the Invention
This invention relates to liquid toners that are useful for electrographic and electrophotographic processes.
2. Discussion of the Art
In electrography and electrophotography a toner is deposited on a charged surface, typically an imagewise charged surface. Electrography generally comprises placing a charge onto selected areas of a dielectric medium with an electrostatic writing stylus or its equivalent to form a charge image, applying toner to the charge image, and fixing the toned image.
Electrophotography forms the technical basis for various well known imaging processes, including photocopying and laser printing. A general discussion of color electrophotography is presented in "Electrophotography, " by R. M. Schaffert, Focal Press, London & New York, 1975, pp. 178-190. The basic electrophotographic process involves placing a uniform electrostatic charge on a photoreceptor element, imagewise exposing the photoreceptor element to light thereby dissipating the charge in the exposed areas, developing the resulting electrostatic latent image with a toner, and transferring the toner image from the photoreceptor element to a final substrate, such as paper, either by direct transfer or via an intermediate transfer material. Since the toner image has to undergo at least one and usually two transfers, it would be highly desirable to develop a toner which displays good differential adhesion, i.e. releases well from the photoreceptor element and adheres strongly to the final receptor.
Liquid toners are well known in the imaging arts. See, for example, Schmidt, S. P.; Larson, J. R.; Bhattacharya, R. in Handbook of Imaging Materials, Diamond, A. S., Ed.: Marcel Dekker, New York, 1991, pp. 227-252 or Lehmbeck, D. R. in Neblette's Handbook of Photography and Reprography, Sturge, J., Ed.: Van Nostrand Reinhold, New York, 1977, Chapter 13, pp. 331-387. A liquid toner is a dispersion of colloidal size particles, hereinafter referred to as "toner particles", in a carrier liquid (or dispersing medium) which has a low dielectric constant. The toner particles comprise a colorant and a film-forming resin and carry an electrostatic charge. The toner particles in the dispersion are capable of migrating under the influence of an electric field and being deposited on a surface bearing an opposite charge, thereby forming an image.
Typically, the carrier liquid is a hydrocarbon that has a low dielectric constant (e.g., less than 3) and a vapor pressure sufficiently high to ensure rapid evaporation of solvent following deposition of the toner onto a photoreceptor, transfer belt, and/or receptor sheet. Rapid evaporation is particularly important for cases in which multiple colors are sequentially deposited and/or transferred to form a single image. An example of such a carrier liquid is the ISOPAR.sup..TM. family of solvents (boiling point range: 130-160.degree. C.).
Both dyes and pigments have been used as the colorant in toner particles. The primary advantages of dyes are their bright colors and transparency. One of the advantages of pigments is that color migration, or "bleeding", is minimized at the fusion step. Another advantage of pigments as colorants for toners is the durability of the color and high optical densities of the image.
In addition to the colorant, the toner particles include a resin. The resin stabilizes the dispersion of the toner particles in the carrier liquid by means of the solvated floating chains of the resin as well as by electrostatic charges which could be imparted to the resin either by metal ion complexation or the chemical structure of the resin. The resin assists in film formation of toner deposited on a receptor by the coalescence of the toner particles and the fixing of the pigment particles. In addition, a resin which provides the toner film with differential adhesion properties with respect to the substrate surfaces is usually desired. Finally, the glass transition temperature (T.sub.g) and the morphology of the resin control other toner film properties such as scratch resistance and overprintability of different colors.
Use of organosols as all or part of the resin component of the toner particles is known. Organosols usually comprise a thermoplastic resinous portion, which is not highly soluble in the carrier liquid, chemically anchored to steric stabilizing polymers or copolymers which are soluble in the carrier liquid. When dispersed in the carrier liquid, the solubility differences of these two portions of the organosol cause it to have a core-shell or "micelle" type of structure, where the substantially insoluble thermoplastic resin forms a "core" which is surrounded by the "shell" of soluble polymer or copolymers. When the carrier liquid evaporates, either the core or shell properties can dominate the physical properties of the resulting film.
Toner particles are typically made by a process having the steps of polymerizing the resin or binder (this step typically occurs in a solvent), milling the resin with pigment particles, drying the milled mixture, grinding the dried mixture to form colloidal sized particles and suspending the particles which are a composite of pigment and resin in the dispersing liquid.